Net community production, nutrients, and hydrographic parameters in the South China Sea in summer 2017

In summer, the Vietnam Offshore Current (VOC) and the Kuroshio intrusion are two important processes provoking considerable environmental fluctuations in the South China Sea (SCS). Net community production (NCP) is an important proxy of biological pump strength and can be estimated based on the dissolved oxygen to argon ratio (O2/Ar) in the mixed layer. To determine the influence of the VOC and Kuroshio intrusion on the NCP in the oligotrophic SCS, we conducted high-resolution underway measurements of O2/Ar and hydrographic parameters using membrane inlet mass spectrometry (MIMS, HPR-40, Hiden, UK) and multi-parameter water quality logger (RBR Maestro, Canada) during the cruise in the northeastern SCS in summer 2017. NCP in the mixed layer was estimated using the supersaturation of O2/Ar (Delta O2/Ar) and gas transfer velocity (k). All the underway observation data were compiled into the 5-min interval. To monitor the nutritive fluctuations induced by the VOC and Kuroshio intrusion, we also collected surface water samples from Niskin bottles at sampling stations for the nutrients analysis; the nutrients were then determined by an auto-analyzer. We divided the cruise into three phases (Phase 1, 2, and 3); Phase 1 was dominated by the Kuroshio intrusion, while Phase 3 was influenced by the VOC. Because the upwelling driven by cyclonic eddies and typhoons could introduce considerable uncertainties to the NCP result, we excluded the data obtained in the upwelling regions.